The invention relates to a method and composition for selectively reducing the permeability to water of hydrocarbon reservoirs which are hot and saline. It selectively reduces circulation of water in underground formations without thereby reducing the passage of oil or gas. The invention is of particular interest when the water is hot and/or saline, for example at a formation temperature of 70.degree. to 130.degree. C. and/or a water salinity at least as great as that of sea water. It is particularly applicable to assisted recovery of hydrocarbons.
The recovery of liquid and gaseous hydrocarbons from underground formations is very often accomplished by the production of large quantities of water. In some cases, although significant hydrocarbon production is obtained, so much water is produced and the cost of treating it is so high that hydrocarbon production becomes uneconomic. More specifically, the massive inflow of water increases both the costs of production, for example by gas lift, and the costs of separating the fluids for treating the water and discharging it. It also prevents the hydrocarbon from flowing to the well and aggravates problems of sediment formation and corrosion. In heterogeneous reservoirs, excessive water production is often caused by digitation of the water injected through the zones of high permeability. This leads to a premature breakthrough of water into the production well, to bad volumetric scavenging and finally to inefficient hydrocarbon recovery.
Many methods of reducing the permeability to water of very permeable formations have been proposed and tested in situ; they generally consist of putting either a cement or a suspension of solid particles or paraffins into the formation, at the level of the zone to be insulated. Resins or gels of water soluble polymers have more recently been proposed and used. All these methods have the drawback of not being selective and of blocking the circulation of oil or gas almost as much as that of water.
The use of water soluble polymers of high molecular weight in the absence of any cross-linking or curing agent has been proposed more recently. Compared with previous solutions and particularly those using resins or polymer gels, these polymers have the advantage of reducing water circulation without adversely affecting oil or gas production.
Two families of water soluble polymers of high molecular weight have been used in particular; hydrolyzed or non-hydrolyzed polyacrylamides and some polysaccharides, particularly non-ionic polysaccharides.
Thus, U.S. Pat. No. 4,095,651 recommends using hydrolyzed polyacrylamides of high molecular weight dispersed in a water more saline than that of the formation, as a means of selectively combatting the irruption of water from the producing wells. However, this method is of limited usefulness since it is effective chiefly in reducing irruption of water of low salinity (.ltoreq.5 g/l of total salts dissolved) and because hydrolysis of the polyacrylamide to polyacrylate reduces its effectiveness as soon as the temperature rises above 60.degree.-70.degree. C.
U.S. Pat. No. 4,842,071 recommends simultaneous or consecutive injection of a non-ionic polymer or copolymer of acrylamide and of a basic agent as a means of selectively reducing irruption of water into producing wells. Although this method is effective in reducing the irruption of very salt water, it has the same disadvantages as those based on hydrolyzed polyacrylamides as soon as there is a high temperature at the bottom.
U.S. Pat. No. 4,155,405 recommends injecting a non-hydrolyzed aqueous polyacrylamide gel and a dialdehyde into wells which have already been sunk or are in the process of being sunk, in order to prevent water from irrupting. Apart from the fact that the stability of the basic polymer is reduced when the temperature of the reservoir rises, the gel which is formed in situ is not selective and therefore blocks both the water and the oil or gas.
U.S. Pat. No. 4,133,383 describes a process for definitively blocking fluid flow in a formation at the level of a well with non-controlled production, in which a non-viscous fluid in an acid medium is injected at low temperature through a secondary reservoir. The non-viscous fluid comprises a polysaccharide cross-linked by a dialdehyde, which is converted by the effect of temperature to a highly viscous fluid in the formation and in the vicinity of the non-controlled well. The serious disadvantage of this type of process is that it definitively stops any production of fluids at the level of the production well.
Assignee's U.S. Pat. No. 4,718,491 recommends using various polysaccharides and particularly a scleroglucan, in the absence of any stabilizing additive for selective reduction of permeability to water in the vicinity of an oil or gas producing well. The preferred field of application for the process is in the production of hot water (up to 130.degree. C.) and salt water (salinity over 30 g/l).
However, it is found that the polymer does not remain stable very long in a porous medium, particularly at high temperature. Thus at about 75.degree. C., in the presence of salt water, the polymer has been found to have satisfactory stability for a period of a year, whereas at about 120.degree. C. one cannot generally hope to stabilize the polymer longer than a week without any additives.